The present invention relates to a lapping oil composition which is advantageously used in finish-grinding of a material to provide a high-quality grinding surface. The present invention particularly relates to a lapping oil composition which is advantageously used in finish-grinding in which a composite material composed of a plurality of materials having different hardness from each other is uniformly ground to provide a high-quality grinding surface, without causing difference in the grinding amount between soft materials and rigid materials, i.e. selective grinding, which is generally caused during lapping and polishing processes of the composite material. The present invention further relates to a lapping oil composition which is advantageously used in finish-grinding in which a composite material is uniformly ground to provide high-quality grinding surface, without causing difference in the grinding amount between different materials, i.e. selective grinding, said finish-grinding using a lap liquid containing no abrasive grains which is conducted after the grinding processing of a surface of a thin film magnetic head to be an air bearing surface, using free abrasive slurry.
Recently, higher performance and higher function have been more and more demanded for optical parts, electronic parts, precision machine parts or the like, and wide range of materials have been used for such parts, such as metallic crystalline materials, ceramics, glass, plastics and so forth.
As one of the manufacturing processes of such parts, grinding or polishing of a composite material composed of a plurality of materials having different hardness from each other is frequently introduced. Recited as examples of grinding processing of composite materials are: in the field of electronics, texturing of Nixe2x80x94P plating of a hard disk substrate, uniform working of wiring metal layers and insulation films between the layers in multiple layer wiring process of LSI; and in the field of optics, grinding of the connector end faces of optical fibers consisting of a composite material composed of zirconia ceramics (so called xe2x80x9cferrulexe2x80x9d), quartz glass (so called xe2x80x9ccorexe2x80x9d of fiber) and fluoroplastics (so called xe2x80x9ccladxe2x80x9d).
With respect to a hard disk drive (i.e. a recording medium of computer), the packing density has been increased year by year. As means to attain a higher packing density, the gap or spacing between the hard disk and a magnetic head has been reduced. In other words, reduction of the spacing of the raised head has been attempted.
A magnetic head mounted on a hard disk drive is generally of a thin film type magnetic head type, and examples of this type are of an inductive type, an MR-inductive complex type wherein MR (magnet resistance) is used as a recording/reproducing element, and a type using GMR (Giant MR).
These thin film type magnetic heads are composed of a composite material comprised of a substrate such as Altic (Al2O3xe2x80x94TiC), a ceramic protective/insulation film such as alumina (Al2O3), a metallic film which is a magnetic material such as permalloy (Fexe2x80x94Ni) and Sendust (Fexe2x80x94Alxe2x80x94Si) and the like.
For example, a thin film type magnetic head 12 shown in FIGS. 1 and 2 comprises an Altic substrate 1, an alumina insulation film 2, a bottom shield film 3 (Sendust: FeAlSi, permalloy: Fexe2x80x94Ni or the like), an alumina film 4, an MR element 5, an alumina film 6, a head shield film 7 (permalloy or the like), an alumina film 8, a write pole tip 9 (permalloy or the like), an alumina protective film 10, and a coil conductor 11.
When conventional free abrasive slurry is used for grinding of ABS (Air Bearing Surface) of a thin film type magnetic head, in most cases, stepped or rough surface results due to selective grinding or abrasion of a metallic film made of soft materials such as permalloy and Sendust, due to difference in hardness between the materials. As a result, there is a problem in that the metallic film (such as magnetic pole portion) is recessed from ABS composed of ceramics, which is called PTR (Pole Tip Recession), increasing the magnetic spacing to a recording medium (as shown in FIG. 2), thereby leading to substantial increase in raised or floating distance of the head.
In order to avoid the above-mentioned selective grinding or damage, Japanese Patent Application Kokai Nos. 3-92264 and 9-245333 proposed that finish-grinding using a lap liquid containing no abrasive grains be conducted, after ABS grinding using free abrasive slurry.
Introduction of such finish-grinding has remarkable effect on solving the above-mentioned problems, in other words, by conducting grinding processing using a lap liquid containing no abrasive grains, selective grinding or rough surface caused by ABS grinding using free abrasive slurry can be recovered.
In addition, an invention which provides free abrasive slurry itself with an ability to avoid selective grinding has been made. For example, Japanese Patent Application No. 10-113327 discloses that, polyether having a molecular weight of 300-20,000 and having 1-6 hydroxide functional groups, obtained by addition reaction of propylene oxide, or optionally ethylene oxide, is added as an anti-selective grinding agent to free abrasive slurry, thereby avoiding selective grinding caused during lapping processing of ABS of a thin film type magnetic head as well as improving the quality of the grinding surface.
Japanese Patent Application No. 10-255022 describes the use of a sulfur-containing organic molybdenum compound as an anti-selective grinding agent. The compound is decomposed by frictional heat generated during grinding processing and forms a film without causing any reaction with the metal surface. The film has the layered structure wherein the layers are mainly composed of molybdenum disulfide (MoS2) and bonded to each other by Van der Waals force which is a weak intermolecular force. Friction at the contact area is replaced by interlayer friction in molybdenum disulfide and is lowered. As a result, the difference in the grinding amount between materials of various hardness is reduced and selective grinding of soft materials is effectively avoided.
However, reduction of the spacing of the raised head and lowering of the PTR value has been further demanded, while improving the quality of grinding surface. Therefore, selective grinding of the metallic film should be avoided, and at the same time, step difference (so called xe2x80x9cshoulder step differencexe2x80x9d) formed at the boundary between Altic (a substrate) and alumina (an insulation film) during grinding processing using free abrasive slurry should be lessened. In other words, further lowering of the PTR value and the improving grinding surface quality become difficult using conventional methods.
Problems to be solved by the present invention
The present invention relates to a lapping oil composition which is advantageously used in finish-grinding of a material to provide a high-quality grinding surface. The present invention particularly relates to a lapping oil composition which is advantageously used in finish-grinding in which a composite material composed of a plurality of materials having different hardness from each other is uniformly ground to provide a high-quality grinding surface, without causing difference in the grinding amount between soft materials and rigid materials, i.e. selective grinding, which is generally caused during lapping and polishing processes of the composite material. The present invention further relates to a lapping oil composition which is advantageously used in finish-grinding in which a composite material is uniformly ground to provide high-quality grinding surface, without causing difference in the grinding amount between different materials, i.e. selective grinding, said finish-grinding using a lap liquid containing no abrasive grains which is conducted after the grinding processing of a surface of a thin film magnetic head to be an air bearing surface, using free abrasive slurry.
Accordingly, the object of the present invention is to provide a lapping oil composition and method for finish-grinding method which overcomes the above-mentioned problems.
The present invention provides a lapping oil composition for finish-grinding comprising a non-aqueous solvent, at least one member of acetylene glycol compounds and optionally at least one member of phosphoric ester compounds.
The present invention specifically provides the above-mentioned lapping oil composition, wherein said acetylene glycol compound is represented by the following formula (I):
R1C(R2)(O(CnH2n+1O)mH)Cxe2x89xa1Cxe2x80x94C(O(CnH2n+1O)mH)(R3)R4 xe2x80x83xe2x80x83(I) 
where each of R1, R2, R3 and R4 independently represents an alkyl or alkylaryl group, n=2-4 and m=0-6;
and the above-mentioned lapping oil composition, wherein said phosphoric ester compound is represented by the following formula (II):
[Rxe2x80x94Oxe2x80x94(CnH2n+1O)m]x(OM)3xe2x88x92xPxe2x95x90O xe2x80x83xe2x80x83(II) 
where R represents a C8-C18 alkyl, alkenyl, alkynyl, aryl or alkylaryl group, n=2-4, m=0-4, x=1-2 and M represents H, Na, K, Ba, an amine, such as NH4, or an alkanolamine, such as NH(C2H4OH)3; and the above-mentioned lapping oil composition, which is advantageously used in finish-grinding using a lap liquid containing no abrasive grains which is conducted after the grinding processing of a thin film magnetic head using free abrasive slurry.
Due to the combination of the carbon-carbon triple bond which locates at the center of acetylene glycol compound to be used in the present invention and a hydroxyl or alkoxyl group adjacent to the carbon, pi electron density is remarkably enhanced, and the center of acetylene glycol molecule shows strong polarity.
Therefore, during finish processing, pi electrons of the triple bond and terminal hydroxyl groups (polar groups) of the acetylene glycol compound are oriented to the metallic film of the magnetic head, and selectively adsorbed to the film, thereby forming a protective film, which prevents the metallic film surface from scratching.
According to classification based on acid-base property, metals such as permalloy is classified as soft acid. Since the phenomenon of adsorption is one type of acid-base interaction, it is known that the similar type of acid and base tend to react with each other.
Based on this principle, soft base has high adsorption ability to metal (soft acid), and this mechanism is believed to be the reason for the selective adsorption of the acetylene glycol compound having pi electron to the metallic surface.
On the other hand, newly formed surface obtained by grinding of ceramics, such as Altic and alumina, is uneven and known to have points having different degree of activities from each other (see Masayuki Mori: Tribologist 36-2 (1991) 130-134).
It is believed that phosphoric ester compound has a function of reducing the step between the Altic and alumina owing to the fact that phosphoric ester compound is strongly adsorbed by active points on the newly formed surface of the ceramics, rather than by newly formed surface of the metals, due to its strong acidity, and forms a film. In other words, since ionic bond is predominant between the atoms on the surface of the ceramics such as metal oxide and the crystalline surface is covered with polarized oxygen atoms having larger diameter than that of cations such as metal ions, it is expected that the acidic phosphoric ester compound, which is ionic, is easily adsorbed by the surface of the ceramics.
In the case of the lapping oil of the present invention which contains an acetylene glycol compound, more preferably an acetylene glycol compound and a phosphoric ester compound, it is expected that selective grinding of metallic film be lowered, because the acetylene glycol molecule is selectively adsorbed to the surface of metallic film composed of permalloy or the like, due to the coordination effect by the pi electron of the triple bond and the effect of the terminal hydroxyl groups (polar groups) present in the acetylene glycol molecule. On the other hand, the phosphoric ester compound is electrostatically adsorbed to the ceramic surface due to its strong acidity. Namely, the acetylene glycol compound is selectively adsorbed to the metallic film composed of permalloy or the like, and the phosphoric ester compound to the ceramic surface composed of Altic, alumina or the like, thereby avoiding selective grinding between altic/alumina/metallic film, having different hardness from each other, resulting in excellent ability to attain low PTR value and high grinding surface quality.